Opal oleate is a pigment product currently utilized for toning carbon black for use in typewriter ribbons and computer tape, for example. Opal oleate is prepared by reacting opal with oleic acid according to various methods. The structural formula for opal oleate is as follows: ##STR1## wherein R.sub.1, R.sub.2 and R.sub.3 are C.sub.6 H.sub.5, and X.sub.1, X.sub.2 and X.sub.3 are H.
The opal itself (e.g. Spirit Blue C.I. 42760) from which opal oleate is derived is prepared by phenylating pararosaniline base with aniline using glacial acetic acid as a catalyst. Opal primarily consists of triphenylated pararosaniline or TPRR, whose structural formula is as follows: ##STR2## wherein R.sub.1, R.sub.2 and R.sub.3 are C.sub.6 H.sub.5, and X.sub.1, X.sub.2 and X.sub.3 are H.
The general formula for the triphenylmethane compound is: ##STR3## wherein R.sub.1, R.sub.2, and R.sub.3 are direct linkages, phenyl or naphthyl groups; R.sub.4 is hydrogen or methyl; X.sub.1, X.sub.2, and X.sub.3 are hydrogen, halogen alkyl or alkoxy of 1 to 4 carbons; provided that at least one of R.sub.1, R.sub.2 and R.sub.3 is not a direct linkage and provided that X.sub.1, X.sub.2 or X.sub.3 is hydrogen when the corresponding R.sub.1, R.sub.2 and R.sub.3 is a direct linkage.
The reaction for the preparation of opal is conducted in aniline which is distilled off at the end of the reaction, thereby isolating the opal as a solid. The hue of opal depends on the degree of phenylation and typically varies from red to green. Lower codes of opal (such as OB-30 and OB-60) normally contain higher, e.g. 5-10%, levels of aniline after reaction. It is difficult to remove all the aniline from the opal product by physical means because opal has a high affinity for aniline.
Unfortunately, the aforesaid levels of aniline remaining in the opal product, and subsequently in the opal oleate prepared with the aforesaid opal, are currently viewed as unsatisfactory. This is because aniline is a relatively toxic substance and is suspected of being a carcinogen.
Process development work was conducted in the early 1980's to reduce the level of aniline in opal by vacuum distillation using a wiped film evaporator. The effects of various process parameters such as feed rate, opal concentration in the feed, jacket temperature and degree of vacuum were studied to optimize the operation from the standpoint of throughput and aniline content in the product. In most instances, opal could be produced with a residual aniline content of 4-7%. At one point, the aniline content was reduced to about 2% or higher, but the opal product was extremely difficult to flake and could not be used for subsequent conversion to opal oleate. This development work failed to produce a workable opal product with less than 0.5% aniline.
There currently exists a need in the art to produce opal and opal oleate with extremely low levels of aniline (about 500 parts per million (p.p.m.) or less) for applications in the specialty colorants field. Preparation of substantially "aniline free" opal and subsequently its oleate have remained a goal of those skilled in the art.